The invention relates to screen printing methods in general, and more particularly to improvements in screen printing methods which can be carried out with flat bed screen printing machines. Still more particularly, the invention relates to improvements in screen printing methods for the application of paste (such as printing ink) to flexible or rigid materials with a high degree of accuracy such as normally requires the application of so-called passer markers or marks and monitoring of the applied passer marks by detectors in order to ensure highly accurate positioning of selected portions of the material to be printed with reference to the stencil.
Commonly owned Klemm U.S. Pat. No. 4,510,864 discloses a screen printing machine wherein a flexible web to be printed is transported in two stages including a main stage for imprinting and a second stage for reading passer marks. An advantage of such machines is that the application of liquid material can be carried out with a high degree of accuracy. The reason is that the detector of passer marks ensures highly accurate positioning of a selected portion of the web preparatory to printing, and that the web portion which is to be printed lies absolutely flat. However, such high degree of accuracy is achieved at the expense of limiting the output of the machine. The web is advanced by an intermittently driven heated roller which can be rotated at a reasonably high speed during the main stage but is rotated very slowly during the second stage which follows the main stage and consumes an inordinately large part of the total time which elapses for completion of a main stage and a second stage. Moreover, the web is idle in the course of the actual printing operation.
Published German patent application No. 31 36 175 discloses a screen printing machine with a counterpressure roller in lieu of a flat bed. The web is in motion during application of paste. The screen is located at a level above the counterpressure roller and is movable back and forth in and counter to the direction of forward movement of the web. The squeegees are stationary, i.e., the screen can move back and forth with reference to the squeegees. The position of the web (in order to move a passer mark to an optimum position) is corrected while the so-called flood bar or drag squeegee is in the process of flooding the pool of liquid medium and while the screen is in motion in the direction of forward movement of the web. This is possible because the screen is free to move relative to the counterpressure roller. The machine which is disclosed in this published German application operates intermittently in that it carries out a so-called two-stage operation, the same as the machine of Klemm U.S. Pat. No. 4,510,864. The counterpressure roller is mounted for rotation about a fixed axis, i.e., it cannot move in and/or counter to the direction of advancement of the web, the same as the flat bed of the machine in the patent to Klemm. Though the machine of the published German patent application is faster than the machine of Klemm, it cannot print with a degree of accuracy which can be achieved with a screen printing machine employing a flat bed.